Not applicable.
The present invention relates generally to subsea pig launching, and more particularly to a method and apparatus for storing and sequentially launching multiple pigs from a remote location. Still more particularly, the present invention is a method and apparatus for delivering pigs to a remote subsea pig launching apparatus.
In the oil and gas industry, it is common to remove petroleum deposits and other debris from pipelines by pushing a cylindrical scraper, referred to as a xe2x80x9cpig,xe2x80x9d through the lines using fluid pressure. The pig is introduced into the pipeline from a launching system that typically includes a releasable retainer for retaining the unlaunched pig and a source of fluid pressure connected to the housing behind the pig.
Subsea satellite wells, manifolds or templates in deepwater are typically connected to a host platform located in shallow water via subsea flowlines that transport the produced hydrocarbon fluid along the sea floor. Such systems are often referred to as xe2x80x9ctie-backxe2x80x9d systems. As with other types of pipelines, flowlines in tie-back systems need to be pigged periodically during their operation to remove paraffin deposits, displace liquids, etc. The traditional method of pigging such flowlines has been to use xe2x80x9cround trip pigging,xe2x80x9d which requires a pair of parallel flowlines between the host platform and the manifold or the wellhead. Pigs are typically launched from and received at the host platform, traveling outward through one flowline and returning through the other.
A more economical option for deepwater flowlines is to use xe2x80x9csingle trip piggingxe2x80x9d using a single flowline between the manifold and the host platform. In single trip pigging, the pigs are launched from a pig launcher mounted on the subsea manifold and received on the host platform. The pig launcher can also be mounted on a wellhead or a pipeline end manifold (PLEM) sled, which is connected to production manifold/wellhead via jumpers.
In order to maximize the advantage of a single trip pigging system, the cost of offshore intervention in the system should be minimized. Such intervention is required either to replace the pigs in the launcher or to retrieve the launcher and recharge it with a new set of pigs after the previous set has been launched into the flowline. This implies that the pig launcher should be able to hold a large number of pigs and, for deepwater application, all pigging operations should be performed using an ROV or remotely from the host platform. Hence, it is desired to provide a reloading apparatus that can be used in conjunction with an ROV and can carry and deploy several pigs into a subsea pig launching apparatus.
Furthermore, depending on the pigging operation frequency, the pigs are likely to be left in the launcher over a long period, as much as one to two years. However, since conventional pigs tend to degrade when exposed to hydrocarbons or methanol for a long period, conventional pigs cannot be left exposed to hydrocarbons or methanol while they are stored in the pig launcher. Hence, it is further desired to provide a pig launching system that does not store the pigs in either production fluids or methanol.
The present invention provides an apparatus for launching pigs into a subsea flowline that connects a subsea manifold, template or a wellhead to a host platform and provides a reloading apparatus that can be used in conjunction with an ROV and can carry several pigs and deploy them remotely into a subsea pig launching apparatus.
The subsea pig launcher described herein addresses the issues identified above. While the subsea pig launcher is described in the following discussion as being installed on a manifold, the present apparatus can be easily adapted for other applications, such as pigging from a subsea wellhead, template or a pipeline end manifold (PLEM) sled at the end of flowline.
More particularly, a method and apparatus have been developed for loading multiple pigs into a subsea pig launcher barrel, storing them inside the barrel over an extended period of time and then releasing the pigs remotely, one at a time, into a subsea flowline or pipeline as needed. The inside diameter of the launcher barrel is preferably slightly larger than the outside diameter of the pigs, so as to allow easy movement of pigs inside the barrel. Since the pigs are pushed inside the barrel by a positive fluid pressure, the system can operate even with the barrel made to fit the pig outside diameter. The top or rearmost pig is constructed so that its outside diameter seals against the inside wall of the barrel, and is therefore referred to as the xe2x80x9cpiston pig.xe2x80x9d
The pigs are advanced inside the barrel by providing hydraulic fluid under pressure behind the piston pig. This advances the stored pigs until the foremost pig is adjacent to a chamber called the xe2x80x9cpig parking chamberxe2x80x9d at the bottom of the barrel. The entrance of the pig parking chamber can be opened to allow the foremost pig to enter the chamber. In the chamber, the foremost pig is held between two pig stops. A kicker line connected to the parking chamber is provided for pumping hydraulic fluid behind the parked pig. By releasing the lower pig stop and pumping hydraulic fluid under pressure through the kicker line, the foremost pig can be launched into another chamber called the xe2x80x9cpig launching chamber.xe2x80x9d The pig launching chamber preferably has an isolation valve at each end, which isolates it from the production header and flowline on one side and the pig parking chamber on the other side. A production kicker line connected to the launching chamber permits introduction of production fluids (hydrocarbons) behind the pig inside the launching chamber. The hydraulic fluid to be used for pushing pigs can be similar to the control fluids conventionally used for subsea systems, or can be any other suitable fluid that has a density greater than seawater, is environmentally acceptable, and is chemically non-reactive with the pig material.
From the launching chamber, the pig is pushed into the production header by opening the isolation valve between the production header and the launching chamber. Once the pig travels from the production header into the flowline, the isolation valve between the launching chamber and the production header is closed. The launching chamber is preferably then flushed with methanol and then, if required, with hydraulic fluid to wash out the residual hydrocarbon fluids.
Additional pigs can be loaded using a pig reloading system. The pig reloading system is adapted to be carried and controlled by an ROV. At the surface, new pigs are loaded into a rack that holds multiple pigs. The pig reloading system, and ROV, are lowered to a subsea pig launcher and attach to a specially designed platform at the top of the barrel. This platform orients the ROV and enables the ROV to maintain the proper position throughout the reloading process. The launcher barrel is opened by the ROV and the pig reloading system uses hydraulic power supplied by the ROV to extend a rack of pigs over the barrel. As the rack extends, the pigs move over the opening of the barrel and are allowed to fall into the barrel. Once the desired number of pigs are loaded, the ROV closes the barrel and returns to the surface. The pig reloading system may be equipped with multiple racks in order to increase the number of pigs loaded during a single trip. The pig reloading system is also preferably equipped with a safety system that ensures that the pig reloading system is fully retracted if hydraulic and electrical control is lost.
The apparatus of the present invention, and in particular the ROV mounted pig reloading system provides several useful advantages over conventional systems. By being able to reload pigs from the surface using only an ROV, the subsea pig launcher can be supplied with pigs using only an ROV support vessel and does not require heavy lifting equipment or a large deck area.